In November 2013 U.S. government agencies, including the Department for Homeland Security, engaged in a massive emergency drill. GridEx II, as it was known, involved thousands of utility employees, the National Guard, the FBI and officials from Canada and Mexico in a simulated failure of large parts of the North American power grid.

GridEx II pitted utilities against a range of attacks designed to be a greater threat than any weather-related incidents. These included denial of service and phishing attacks on corporate networks, cyberattacks on control systems and coordinated physical attacks on key transmission substations and generating facilities that caused widespread damage, including rolling blackouts.

View of the PG&E Metcalf Substation from the City of Santa Clara Motorcycle Park, San Jose, California, January 30th, 2014. The substation was attacked by snipers on April 16th, 2013. They took down more than a dozen big transformers that funnel power to Silicon Valley. Talia Herman for The Wall Street Journal

The attack at the PG&E Corp. facility was “the most significant incident of domestic terrorism involving the grid that has ever occurred” in the U.S., said Jon Wellinghoff, who was chairman of the Federal Energy Regulatory Commission at the time.

“This wasn’t an incident where Billy-Bob and Joe decided, after a few brewskis, to come in and shoot up a substation,” according to Mark Johnson, retired vice president of transmission for PG&E. “This was an event that was well thought out, well planned and they targeted certain components.”

Snipers, shooting for 19 minutes, surgically knocked out 17 giant transformers that funnel power to Silicon Valley. Mr. Wellinghoff, in later comments to the Washington Post, said this sort of attack can cause much more damage with a lot less sophisticated equipment than can be done with [a cyberattack].

There have been many attempted cyberattacks on power infrastructure. The most successful ever carried out was the 2010 Stuxnet virus. This had been developed over many years by Israeli and American intelligence to damage Iran’s nuclear capability. Men armed with AK-47s clearly need less lead time.

But the grid is in fact even more vulnerable than this. The 2003 Northeast blackout, the second largest in the nation’s history, was caused by power lines brushing against trees. The cause of most blackouts is even smaller. Squirrels, it emerges, could be the clearest and most present danger to America’s power.

MARKETS

Crude oil futures Monday were holding on to gains that saw both contracts hit year highs, fueled by continuing U.S. cold weather and uncertainty over the Fed’s future plans. You can read the Journal’s latest oil-markets report here.